PELAGIC PHOTOFERROTROPHS DEPOSIT BIF WITHOUT BIOMASS

Iron-rich, anoxic ancient oceans sustained life on Earth for billions of years. Photoferrotrophs, anoxygenic phototrophs using ferrous iron as the electron donor, were likely the dominant primary producers in these ferruginous oceans. Photoferrotrophs have also been implicated in the deposition of Banded Iron Formations (BIFs), which are massive sedimentary Fe ore deposits. The enigma behind photoferrotrophic BIF deposition is the conspicuous lack of organic matter and/or its diagenetic products that would be expected due to the co-sedimentation of phototrophic biomass and Fe. Such co-sedimentation is indeed observed during Fe oxidation by extant photoferrotrophs studied in the lab to date. We have isolated the first pelagic photoferrotroph from Kabuno Bay, East Africa. This isolate, tentatively named Chlorobium ferrooxidans str. Kabuno Bay (KB), has the remarkable capacity to shed its ferric iron metabolites. In this way, str. KB effectively separates itself from iron and remains buoyant in our lab experiments. This metabolic strategy is likely required for a pelagic lifestyle in order to maintain position in the photic zone. Ancient marine photoferrotrophs, like their extant counterparts from Kabuno Bay, would have maximized their access to light by staying buoyant. They therefore would likely have shed spent Fe, effectively separating biomass from phototrophic Fe minerals. Modeling of high productivity coastal upwelling systems, the likely depositional environments for BIFs, shows that this biological effect leads to localized deposition of Fe, but with the large-scale dispersion of biomass. Little biomass would be buried with Fe, explaining the lack of organic matter or its diagenetic products in BIF.